Self-Mobile Space Manipulators

Astronaut extra-vehicular activity (EVA) at a space station is costly,
potentially dangerous, and requires extensive preparation. Some EVA tasks,
such as unplanned repairs, may require the versatility, skill, and on-site
judgment of astronauts. Many other tasks, particularly routine inspection,
maintenance and light assembly, can be done more safely and cost
effectively by robots.

We are developing a relatively simple, modular, low mass, low cost robot for
space station EVA that is large enough to be independently mobile on the
station exterior, yet versatile enough to accomplish many vital tasks.
Because our design is for a robot that is independently mobile, yet capable
of conventional manipulation tasks, we call it the Self-Mobile Space
Manipulator or (SM)2. The robot can perform useful tasks such as visual inspection,
material transport, and light assembly. It will be able to work independently or in
cooperation with astronauts, and other robots.
The research accomplishment we have achieved so far include:

Gravity Compensation System

The zero-gravity environment at an orbiting space station has significant
impact on the design and performance of a robot. The absence of gravitational
forces permits a long, spindly robot to move relatively large masses
with small forces and power consumption. In order to perform
realistic experiments on earth, we have developed a gravity compensation
system that balances the more significant gravitational effects on the
robot so it behaves as if weightless. The gravity compensation system
includes a passive, vertical counterweight
system, and an actively controlled horizontal system. We have currently two
GC systems developed in the lab, the x-y gantry system for the large, global
locomotion experiments and the polar frame system for the fine, local motion
experiments.

5-DOF Flexible Robot Walker

The first version of SM2 robot was designed to have the minimum size and complexity
needed for walking on the space-station trusswork. The basic walker
includes five rotational joints and two slender links.
Grippers at each end of the robot enable it to attach itself to threaded
holes in the truss nodes or other regular structure.
Walking is accomplished by alternate grasping
and releasing of the nodes by the grippers, and swinging of the feet
from one node to the next. During each walking step, one end of the robot
releases from a node, swings 90 or 180 degrees to a desired node
location, and reattaches to that node. SM2 moves along the trusswork
using such steps with alternate feet.

9-DOF Robot Walking Manipulator

The second version of (SM)2 robot was based on the first version as a main frame, and extended the
body to enable manipulation capability. Two additional joints were connected
to each one of the node grippers, and a gripper that we designed and built was
attached to these joints to enable the robot to grasp and carry objects. Two such
2-DOF arm-hand devices assembled at both ends of 5-DOF manipulator provide a 9-DOF manipulator
configuration, while the node gripper remains available for walking.
In addition to its own drive motor, the gripper contains a motorized hex driver
between its jaws to drive the hold-down screw of the ORU mockup.

7-DOF Robot Climber

The third version of (SM)2 was based on the
new design of the international space station alpha whose trusswork is preintegrated,
I-beam shape flanges. The robot now has seven joints: one at the elbow
and three at each end to allow the out-of-plane motions needed for
stepping from one truss face to another, while preserving the robot's
symmetry to simplify control. Joints are self-contained and modular, so a minimum
inventory of parts is needed for joint repair or replacement. The robot links are
long enough to permit climbing between adjacent longerons. At each end of the robot
is a three-fingered gripper for grasping the truss I-beam flanges. Capacitive proximity
sensors at the base of the fingers are used to sense beam proximity, and can be used
for aligning the gripper with the beam.

Space Station Truss Mockup

The space station structural design has
evolved from the original strut-and-node design to the current preintegrated
truss (PIT) design, utilizing aluminum I-beam members. For laboratory experiments, we built
a truss mockup which is a full-scale representation of a small portion
of a truss segment. It includes two longerons, two bulkheads and a diagonal beam, and is
constructed of wood with sheet aluminum laminated to the beam flange faces to provide a
realistic appearance for video images. Having four faces for robot walking
and beam widths of 4.0 and 5.8 inches, the mockup allows a variety
of stepping and grasping motions to verify (SM)2's general motion capability.

Teleoperation Control Station

We have developed a teleoperation control
station including a 6-DOF free-floating, hand-controller, master-slave type walking
demonstrator, data-glove for gesture control, real-time graphics interface for
both display and control, and graphical simulation system for off-line motion
previewing.

Real-time Control System

The control is executed through Chimera real-time
operating system that is developed in the lab. A variety of low-level controllers have
been developed, including acceleration feedback control, adaptive control, and neural
network learning control. Multiple phase control strategies are executed during
different phases of motion and operations. An intelligent control scheme is implemented
for reliable walking using both vision and proximity information for the robot.
Hidden Markov model is used for learning teleoperation for defined tasks and is
implemented for facilitating ORU exchanging tasks.